1. Field
The disclosure relates to the preparation of a silica/natural rubber masterbatch comprising at least a modified silica and a natural rubber latex.
2. Description of Related Art
The term “masterbatch” denotes a composition based on an elastomer in which a filler and possibly other additives have been introduced.
The present disclosure relates in particular to the use of such a masterbatch for manufacturing diene rubber compositions reinforced with an inorganic filler, said compositions being intended for manufacturing tires or semi-finished products for tires, in particular treads for these tires.
To obtain the optimum reinforcing properties conferred by a filler in a tire tread and a high wear resistance, it is known generally to be appropriate for this filler to be present in the elastomeric matrix in a final form which is both as finely divided as possible and as uniformly distributed as possible. However, such conditions can be achieved only when this filler has a very good capability, on the one hand, of being incorporated into the matrix during compounding with the elastomer and of deagglomerating and, on the other hand, of being uniformly dispersed in this matrix.
As is known, carbon black has such capabilities, which is in general not the case of inorganic fillers, particularly silicas. This is because, for reciprocal affinity reasons, these inorganic filler particles have an annoying tendency to clump together in the elastomeric matrix. Such interaction has the deleterious consequence of limiting the dispersion of the filler and therefore the reinforcing properties to a level substantially below that which would be theoretically possible to achieve if all the inorganic filler/elastomer bonds capable of being created during the compounding operation were actually to be obtained. These interactions moreover tend to increase the viscosity in the uncured state of the rubber compositions and therefore to make them more difficult to process than when carbon black is present.
Since fuel saving and the need to protect the environment have become a priority, it has proved necessary to produce tires having a reduced loading resistance without penalizing their wear resistance.
This has been made especially possible by the use, in the treads of these tires, of novel rubber compositions reinforced with inorganic fillers, in particular specific silicas of the highly dispersible type, that are capable of rivalling from the reinforcing standpoint a conventional tire-grade carbon black, while offering these compositions a lower hysteresis, which is synonymous with a lower rolling resistance for tires containing them, and also improved grip on wet, snow-covered or icy ground.
Treads filled with such HD (highly dispersible) silica or HDS (highly dispersible silica) that can be used in tires having a low rolling resistance, sometimes termed “green tires”, due to the saving of energy offered to the user (“green tire concept”), have been abundantly described. The reader may in particular refer to the patent applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99/02590, WO99/02601, WO99/02602, WO99/06480, WO00/05300 and WO00/05301.
These documents teach the use of HD silicas having a BET specific surface area of between 100 and 250 m2/g. In practice, one HD silica having a high specific surface area used in the “green tire” field is particularly the silica “Zeosil 1165 MP” (having a BET surface area of about 160 m2/g) sold by the company Rhodia. The use of this Zeosil 1165 MP silica makes it possible to obtain good compromises in terms of tire performance, especially satisfactory wear resistance and rolling resistance.
The benefit of using silica having a high specific surface area lies mainly in the possibility of increasing the number of silica-elastomer bonds and therefore of increasing the level of reinforcement of the elastomer. This is why it appears advantageous to use, in tire tread rubber compositions, silicas having a high specific surface area, possibly higher than that conventionally used, namely around 160 m2/g, so as in particular to improve the wear resistance of these treads. However, the dispersibility of the filler and the increase in its specific surface area are considered to be conflicting properties. This is because a high specific surface area means an increase in the interactions between filler particles and therefore poor filler dispersion in the elastomer matrix and difficult processing.
Another type of solution for improving the dispersibility of the filler in the elastomer matrix has been envisaged, which consists in compounding the filler and the elastomer in the “liquid” phase. To do so, the process involves an elastomer in latex form, which is in the form of water-dispersed elastomer particles, and an aqueous dispersion of the filler, that is to say a silica dispersed in water, usually called a “slurry”. However, bringing the elastomer latex into contact with the slurry does not allow coagulation within the liquid medium, which coagulation would be necessary in order to achieve a solid which, after drying, would result in the desired silica/elastomer masterbatch being obtained.
This is because silica aggregates are typically hydrophilic in nature and have an affinity with water. Thus, the silica aggregates have a higher affinity with water than with the elastomer particles themselves.
However, various solutions have been proposed for obtaining this coagulation and good dispersion of the filler in the elastomeric matrix in the “liquid” phase via the combined use of an agent for increasing the affinity between the elastomer and the silica, such as a coupling agent, and of an agent, called a coagulant, for producing the coagulation.
Thus, for example, the U.S. Pat. No. 5,763,388 proposes incorporating silica into the rubber latex by treating the silica with a cutting agent and mixing the resulting treated silica with conventional coagulants.
The patent EP 1 321 488 also proposes bringing an aqueous dispersion of negatively charged silica into contact with a diene elastomer latex and with an emulsion containing a polysulfide cutting agent in the presence of a coagulant, such as a polyamine.